1. Field of the Invention
The invention relates to an analog front end (AFE) device with temperature compensation so as to solve a thermal drift due to temperature variation.
2. Description of the Related Art
Traditionally, AFE devices are being applied to two categories of display systems. First, an AFE device applied to a liquid crystal display (LCD) controller without a decoder is used to receive three analog image signals R, G, B from a VGA card of a computer system. Second, an AFE device applied to a video decoder is used to receive a signal from a tuner or a DVD player. Wherein, the signal includes three kinds of video signals as follows. The first is a composite video signal, often called a CVBS signal, which combines the luminance (Y) and chrominance (C) signals into a single channel. The second is a separate video signal separating the luminance (Y) and the chrominance (C) signals. The third is a component video signal which is split into three separate signals Y, Pr, Pb.
FIG. 1 shows a block diagram of a conventional AFE device. An AFE device 100 comprises a bandgap voltage reference circuit 130, a clock generator 140 and one to three identical converting circuits 150. Each converting circuit 150 further comprises a clamper (101, 111, 121), an input buffer (102, 112, 122) and an analog to digital converter (ADC) (103, 113, 123). Take the AFE device in the LCD controller, for example—three converting circuits 150 are required to convert three analog image signals R, G, B into three digital signals D1, D2, D3, respectively. Each converting circuit 150 uses the clamper (101, 111, 121) to calibrate the DC level of the respective analog image signal, then uses the input buffer (102, 112, 122) to buffer the respective analog image signal and finally supplies the respective analog image signal to the ADC (103, 113, 123). The clock generator 140 receives either a horizontal sync (HS) signal or a vertical sync (VS) signal to provide a periodic clock signal to the ADCs (103, 113, 123) for sampling. A reference voltage Vref, generated by the bandgap voltage reference circuit 130, is provided to either the input buffer (102, 112, 122) for making modifications to both a gain and an offset voltage or the ADC (103, 113, 123) for making modifications to a full-scale voltage or a bias current.
In general, the interior of an integrated circuit is divided into a digital circuit and an analog circuit. Normally, there is no thermal drift in the digital circuit. By contrast, the thermal drift could occur in the analog circuit. For example, its voltage varies according to the temperature and its frequency also varies according to the temperature. In applications of display system controllers (including the LCD controllers and the video decoders as mentioned above), users would like the display system to have the same characteristic both at start-up (at a lower temperature) and after warm-up (at a higher temperature), e.g., a consistent display color and a consistent optimum sampling phase of the ADC (103, 113, 123). In other words, it implies that the thermal drift is not allowed to occur in the clampers (101, 111, 121), the ADCs (103, 113, 123), the clock generator 140 and related circuits (e.g., the Sync-on-Green circuit).
Conventional analog circuit designs make use of a variety of techniques, such as generating either a voltage or a current independent of the temperature, to eliminate the thermal drift in the clampers (101, 111, 121), the ADCs (103, 113, 123), the clock generator 140 and related circuits. However, these techniques have little effects on improving the thermal drift and waste hardware resources as well.